Vagary

The vagary is a new enemy introduced in Doom 3. In terms of appearance, she has the lower half of a giant spider and the upper half of a woman. Her abdomen is transparent and seems to contain a malformed mancubus fetus gestating inside. She is first encountered at the end of Alpha Labs Sector 4, serving as the game's first boss fight. However, she is not a unique creature - two more vagaries are encountered in a later level (Caverns Area 2), and another makes an appearance in Hell during the Lost Mission from Doom 3: BFG Edition. She attacks the player with melee strikes and has telekinetic abilities, allowing her to lift various pieces of debris and hurl them at the player.

Tactical analysis
During the first encounter with the Vagary, the player should have both the chaingun and the plasma gun, and both are good weapons to use. The chaingun's damage output is the higher of the two, so is arguably the better choice. The chaingun is capable of killing the Vagary in a little more than the full belt of 60 bullets - possibly a little less than that if headshots are achieved, but her head is fairly small and she moves rather erratically so it is difficult to achieve headshots, and so it may be wiser simply to aim for her body for consistent damage. When the chaingun's belt is nearly empty, it is a valid tactic simply to switch to the plasma gun to finish the job rather than reloading the chaingun.

There is a health station in the room immediately before the Vagary's lair, so the player should have full health before the fight begins. Her melee attacks can be avoided by staying at a distance. Her telekinetic projectiles, however, are difficult to dodge but can sometimes be avoided by sprinting sideways at the right moment. A valid tactic is to move close to the Vagary to provoke her into her melee attack, then darting back out of its reach, and repeat - it takes some practice, but it is arguably easier to avoid damage this way than by attempting to dodge her telekinetic projectiles.

The player should be advised that there are spiky objects on the floor of the arena, possibly the Vagary's egg sacs. These can damage the player if the player stumbles onto them.

If playing on Nightmare difficulty, then the Soul Cube will already be in the player's arsenal and it is wise to have the Soul Cube charged and ready to use against the vagary - it won't kill her in one go, but it will strip most of her health away so she can be finished easily with the chaingun or plasma gun.

Two more vagaries are encountered again much later in the game, in Caverns Area 2 accompanied by a number of trites. This time, however, the player should have the full arsenal of weaponry including the Soul Cube and the BFG. Furthermore, there is much more room available so the player can easily keep their distance to avoid the Vagaries' melee attacks and telekinetic projectiles, and so the battle is less of threat than the encounter in Alpha Labs. It is a fairly straightforward matter to retreat back through the passageways and use the BFG9000 to kill the advancing vagaries and trites. If the BFG9000 is aimed at the vagary, the tracers issued by the projectile will likely kill the nearby trites as well. The Soul Cube can also be launched at a vagary after being charged by killing trites. There is no longer any need to conserve ammo at this point in the game - the next level after this fight is the last, and a large amount of ammo is available upon entering the level.

Another vagary is encountered in Hell in The Lost Mission, this time accompanied by several cherubs. In this encounter there isn't really any debris lying around for the vagary to throw telekinetically, so the player need only keep their distance from her melee attacks and blast away with their weapon of choice. The BFG9000 is a good option since the projectile aimed at the vagary is likely to kill nearby cherubs as well. The player should be careful not to be surrounded by cherubs while focusing on the vagary, however.

Trivia

 * The vagary's spider-like nature is somewhat reminiscent of the Spider Mastermind and also resembles the vore enemy from Quake. Both are likely influenced by Driders, dark elf/spider hybrids in the  universe.
 * In the Making of Doom 3 book, the equation "Sexy + Gross = Creepy" was used to describe the driving concept behind the vagary.
 * Unlike all other bosses from Doom 3, she burns up when she dies and no cutscene is activated upon her death. She is also the only boss monster that appears multiple times. Due to these two features, she could be called a "miniboss". She is also the only female boss in Doom 3, unlike other bosses who are either male or genderless.
 * The vagary is the only boss that is vulnerable to head shots.
 * Although her torso has breasts, these lack nipples.
 * The vagary's upper half has four eyes, being two regular "human" eyes, with other two smaller, spider-like eyes above them, placed on her forehead, at the eyelash level. All the four eyes are pupil-less with black sclera.
 * In Resurrection of Evil, the vagary does not appear, but the trite does.
 * Fans often speculate that it was the vagary who speaks the phrase, "They took my baby," to the marine by using its telepathic power in order to mock or scare him. It could also be speculated that the vagary is what became of the baby that was "taken". The whispering voice is frequently heard when the player gets close to her.